U.S. patent application number 11/446276 was filed with the patent office on 2007-12-06 for adaptive optical transceiver for fiber access communications.
Invention is credited to Wen Li, Qing Zhu.
Application Number | 20070280695 11/446276 |
Document ID | / |
Family ID | 38790338 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070280695 |
Kind Code |
A1 |
Li; Wen ; et al. |
December 6, 2007 |
Adaptive optical transceiver for fiber access communications
Abstract
An optical module includes a transmitter optical sub-assembly
comprising a transmitter configured to emit a
multi-longitudinal-mode (MLM) spectrum signal having an emission
spectrum comprising a plurality of distinct narrow-spectrum peaks
each corresponding to a longitudinal mode in the transmitter. The
emission spectrum can be shifted in wavelength by a change in the
transmitter temperature. The optical module also includes a heating
and cooling device configured to control the temperature of the
transmitter in response to a temperature-control signal and a
receiver optical sub-assembly configured to output a pair of
differential digital signals in response to an input optical
signal.
Inventors: |
Li; Wen; (Fremont, CA)
; Zhu; Qing; (San Jose, CA) |
Correspondence
Address: |
XIN WEN
3449 RAMBOW DRIVE
PALO ALTO
CA
94306
US
|
Family ID: |
38790338 |
Appl. No.: |
11/446276 |
Filed: |
June 2, 2006 |
Current U.S.
Class: |
398/135 |
Current CPC
Class: |
H04J 14/0282 20130101;
H04J 14/0226 20130101; H04J 14/025 20130101; H04B 10/506 20130101;
H04J 14/0246 20130101; H04J 14/0227 20130101; H04J 14/02
20130101 |
Class at
Publication: |
398/135 |
International
Class: |
H04B 10/00 20060101
H04B010/00 |
Claims
1. An optical communication system. comprising; a plurality of
first optical transceiver modules each comprising: a first
transmitter configured to emit a downstream multi-longitudinal-mode
(MLM) spectrum having an emission spectrum comprising a plurality
of distinct narrow-spectrum peaks each corresponding to a
longitudinal mode in the first transmitter, wherein the emission
spectrum of the downstream multi-longitudinal-mode (MLM) spectrum
signal is configured to be shifted in wavelength by a change in the
temperature of the first transmitter: a first temperature
controller configured to control the temperature of the first
transmitter in response to a first temperature-control signal; a
first receiver configured to receive the upstream optical signal: a
first transimpedance amplifier (TIA) coupled to the first receiver,
wherein the first transimpedance amplifier is configured to produce
a first analog photo current monitor signal in response to the
power of the upstream optical signal: and a first post amplifier
coupled to the first transimpedance amplifier, wherein the first
post amplifier is configured to produce a first relative signal
strength indicator (RSSI) signal, wherein the first analog photo
current monitor signal, or the first RSSI signal. or a combination
thereof forms at least a portion of the first power-monitoring
signal: a first wavelength filter cormprising: a plurality of first
branching ports each associated with a specific wavelength channel
wherein each of the first branching ports is in connection with one
of the first optical transceiver modules and is configured to
receive the downstream MLM-spectrum signal from the first
transmitter and send an upstream signal to the first receiver, and
a first common port configured to output a downstream signal in
response to the downstream MLM spectrum signal, wherein the
spectrum of the downstream signal is located in a wavelength
clannel specifically associated with the first branching port at
which the downstream MLM-spectrum signal is received: a second
wavelength filter comprising: a plurality of second branching ports
each associated with a specific wavelength channel, wherein each of
the second branching ports is configured to receive an upstream
MLM-spectrum signal: and a second common port configured to output
the upstream signal in response to the upstream MLM-spectrum
signal, wherein the spectrum of the upstream signal is located in a
wavelength channel specifically associated with the second
branching port at which the upstream MLM-spectrum signal is
received; and a plurality of optical network units each comprising:
a second receiver configured to receive the downstream signal from
the second branching port in connection wtth the optical network
unit: a second transmitter configured to emit the upstream
MLM-spectrum signal to be sent to the second branching port in
connection with the optical network unit. wherein the emission
spectrum of the upstream MLM-spectrum signal comprises a plurality
of distinct narrows-spectrum peaks each corresponding to a
longitudinal mode in the second transmitter, wherein the emission
spectrum of the upstream MLM-spectrum signal is configured to be
shifted in wavelength by changing the temperature of the second
tranmitter; a second temperature controller configured to control
the temperature of the second transmitter in response to a second
temperature-control signal. wherein the downstream signal from the
first transmitter in one of the plurality of first optical
transceivers to the second receiver in one of the plurality of
optical network units and the upstream signal from the second
transmitter in the one of the plurality of optical network units to
the first receiver in the one of the plurality of first optical
transceivers are transmitted in a same wavelength channel: a second
transimpedance amplifier (TIA) coupled to the second receiver,
wherein theL second transimpedance amplifier is configured to
produce a second analog photo current signal in response to the
power of the downstream optical signal; and a second post amplifier
coupled to the second transinipedance amplifier, wherein the second
post amplifier is configured to produce a second relative signal
strength indicator (RSSI) signal, wherein the second analog photo
current monitor signal or the second RSSI signal, or a combination
thereof forms at least a portion of the second power-monitoring
signal.
2. The optical module of claim 1, wherein at least a portion of the
second temperature-control signal is transmitted from the first
transmitter in one of the plurality of first optical
transceivers.
3. The optical module of claim 1, wherein at least a portion of the
first temperature-control signal is transmitted from the second
transmitter in one of the plurality of optical network units.
4. The optical module of claim 1, wherein one of the plurality of
first optical transceiver modules comprises a first temperature
sensor configured to produce at least a portion of the first
temperature-control signal in response to the temperature of the
first transmitter.
5. The optical module of claim 1, wherein one of the plurality of
optical network units comprises a second temperature sensor
configured to produce at least a portion of the second
temperature-control signal in response to the temperature of the
second transmitter.
6. The optical module of claim 1, wherein the spectra of downstream
signal and the upstream signal in the same wavelength channel hiave
substantially the same center wavelength or center wavelengths
offset by one or multiple free spectral ranges (FSRs) of the first
wavelength filter or the second wavelength filter.
7. The optical module of claim 1, wherein the first transmitter is
configured to produce the downstream MLM spectrum signal without
the assistance of an external light source.
8. The optical module of claim 1, wherein the emission spectrum of
the upstream MLM spectrum source is characterized by a center
wavelength that, is configured to be shifted by more than 0.4 nm
for a change of one Celsius degree in the temperature of the second
transmitter.
9. The optical module of claim 1, wherein at least one of the
plurality of first optical transceiver modules comprises a first
micro controller configured to receive a first temperature sensing
signal from the first temperature sensor and a first power
monitoring signal from the first receiver, and to produce the first
temperature-control signal.
10. The optical transceiver module of claim 9, wherein the one of
the plurality of first optical transceiver modules is a unitary
device comprising the first micro controller, the first
transmitter, the first temperature controller, and the first
receiver.
11. The optical transceiver module of claim 1, wherein the micro
controller is configured to receive a first temperature sensing
signal from the first temperature sensor and a first ana1og photo
current monitor signal from the first transimpedance amplifier
(TIA) or the first relative signal strength indicator (RSSI) from
the first post amplifier to produce the first temperature-control
signal, wherein the first analog photo current monitor signal, or
the first RSSI signal, or a combination thereof forms at least a
portion of the second power-monitoring signal.
12. The optical transceiver module of claim 1, wherein the one of
the plurality of first optical transceiver modules is a unitary
device comprising the first micro controller, the first
transmitter, the first temperature controller the first receiver,
the first TIA and the first post amplifier.
13. The optical transceiver module of claim 19. wherein the one of
the plural dv of optical network units is a unitary device
comprising the second transmitter, the second temperature
controller, and the second receiver.
14. The optical transceiver module of claim 1, wherein at least one
of the plurality of optical network units comprises a second micro
controller configured to receive a second temperature sensing
signal from the second temperature sensor and a second
power-monitoring signal from the second receiver, and to produce
the second temperature-control signal.
15. The optical communication system of claim 14, wherein the one
of the plurality of optical network units is a unitary device
comprising the second micro controller, the second transmitter, the
second temperature controller, and the second receiver.
16. The optical communication system of claim 1. wherein the second
receiver is configured to produce at least a portion of the second
temperature-control signal in response to the downstream optical
signal.
17. The optical communication system of claim 1, wherein the first
receiver is configured to produce at least a portion of the first
temperature-control signal in response to the power of the upstream
optical signal.
18. The optical communication system of claim 1, wherein the
plurality of distinct narrow-spectrum peaks in the emission
spectrum of the downstream MLM-spectrum signal are characterized by
an envelope whose full-width at half the maximum is equal to or
greater than 1 nanometer.
19. The optical communication system of claim 1, wherein the
plurality of distinct narrow-spectrum peaks in the emission
spectrum of the upstream MLM-spectrum signal are characterized by
an envelope whose full-width at half the maximum is equal to or
greater than 1 nanometer.
20. The optical commnication system of claim 1, wherein each of the
plurality of first optical transceivers is associated with one of
the plurality of optical network units, and wherein the first
transmitter in one of the first optical transceivers is configured
to tune the second transmitter in the associated optical network
units such that the upstream signal and the downstream signal
between the first optical transceiver and the associated second
transceiver are in a same wavelength channel.
21. The optical communication system of claim 1, wherein a portion
of the second temperature-control signal is transmitted from the
first transmitter in one of the plurality of first optical
transceiver modules, wherein the first transmitters and the second
transmitter associated with the second temperature-control signal
in one of the plurality of optical network units are configured to
transmit optical signals in the same wavelength channel.
22. The optical communication system of claim 1, wherein a portion
of the first temperature-control signal is transmitted from the
second transmitter in one of the plurality of optical network
units, wherein the second transmitters and the first transmitter
associated with the first temperature-control signal in one of the
plurality of first optical transceiver modules are configured to
transmit optical signals in the same wavelength channel.
23. The optical communication system, comprising: a first optical
transceiver module, comprising: a first transmitter configured to
emit a downstream optical signal without the assistance of an
external light source, wherein the downstream optical signal
comprises a first emission spectrum that is configured to be
shifted in wavelength in response to a chance in the temperature of
the first transmitter: a first temperature controller configured to
control the temperature of the first transmitter in response to a
first temperature-control signal; and a first receiver configured
to output a first digital electronic signal in response to an
upstream optical signal, wherein the first receiver is configured
to produce at least a portion of the second temperature-control
signal in response to the power of the upstream optical signal; and
a second optical transceiver module. comprising: a second
transmitter contrenred to emit the upstream optical signal having a
second emission spectrum that is configured to be shifted in
wavelength in response to a change in the temperature of the second
transmitter, wherein the second transmitter is configured to emit
the upstream optical signal without the assistance of an external
light source: a second temperature controller configured to control
the temperature of the second transmitter in response to a second
temperature-control signal produced by the first transmitter: and a
second receiver configured to output a second electronic signal in
response to the downstream optical signal, wherein the second
receiver is configured to produce at least a portion of the first
temperature-control signal in response to the power of the
downstream optical signal.
24. The optical communication system of claim 23, wherein the first
optical transceiver module further comprises: a transimpedance
amplifier (TIA) coupled to the first receiver, wherein the
transimpedance amplifier is configured to produce a photo current
monitor signal in response to the power of the upstream an optical
signal, and a post amplifier coupled to the transimpedance
amplifier, wherein the post amplifier is configured to produce a
relative signal strength indicator (RSSI) signal, wherein the photo
current monitor signal, or the RSSI signal, or a combination
thereof forms at least a portion of the first power-monitoring
signal.
25. The optical communication system of claim 2, wherein the first
optical transceiver module is a unitary device comprising the first
transmitter, the first temperature controller, the first receiver,
the first TIA, and the first post amplifier.
26. The optical communication system of claim 23, further
comprising: a first temperature sensor configured to output a first
temperature sensing signal in response to the temperature of the
first transmitter, wherein the first temperature-control signal
comprises the first temperature sensing signal.
27. The optical communication system of claim 23, further
comprising: a second temperature sensor configured to output a
second temperature sensing signal in response to the temperature of
the second transmitter, wherein the second temperature-control
signal comprises the second temperature sensing signal.
28. The optical communication system of claim 23. wherein at least
a portion of the second temperature-control signal is received from
the second receiver in response to a downstream temperature-control
optical signal produced by the first transmitter
29. The optical communication system of claim 28. wherein the first
transmitter is configured to produce the downstream
temperature-control optical signal in response to the upstream
optical signal from the second transmitter to the first receiver.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application is related to commonly assigned U.S. patent
application Ser. No. 11/396,973, titled "Fiber-to-the-premise
optical communication system" by Li et al, filed Apr. 3, 2006, and
U.S. patent application Ser. No. 11/413,405, titled "High speed
fiber-to-the-premise optical communication system" by Li et al,
filed Apr. 28, 2006. The content of these disclosures is
incorporated herein by reference.
BACKGROUND
[0002] The present disclosure relates to optical communication
technologies.
[0003] As the Internet, voice over Internet Protocol (VoIP), and
Internet Protocol television (IPTV) grow in popularity, more and
more users desire to have accesses to these services from their
premises. The most common local network accesses to these services
are the digital subscriber line (DSL) and the cable modem. The DSL
and cable networks respectively operate on a pair of copper wires
or coaxial cable. While the DSL and the cable modem allow data
transfer at up to several million bits per second downstream to a
user, the upstream data transfer is usually at lower transfer
rate.
[0004] Passive optical network (PON) is attractive network
architecture for the last-mile access because it does not require
active components for directing optical signals between a central
office and the network subscribers' terminal equipment. The PON can
be divided into three main categories: time division multiplexing
(TDM), wavelength division multiplexing (WDM), and a combination of
TDM and WDM.
[0005] Fiber to the premises (FTTP) is a desirable architecture for
providing access from the users' premises. FTTP takes optical
fibers all the way into the users' homes or premises. Currently,
time-division-multiplexing passive optical networks (TDM-PON) are
the primary deployment methods for FTTP. TDM-PON is a
point-to-multipoint architecture utilizing an optical power
splitter at a remote node. TDM-PON delivers downstream information
through broadcasting and bandwidth sharing, and receives upstream
information via time division multiple access (TDMA). One drawback
with TDM-PON is associated with the low security of the broadcasted
downstream signals. The complexity of the TDMA protocols also makes
TDM-PON undesirable for the next generation user-centric
high-speed, broadband services.
[0006] A recent development in the FTTP is PON based on wavelength
division multiplexing (WDM) technology. FIG. 1 illustrates a
conventional WDM-based optical network 100 that includes a pair of
WDM filters 108 and 116 for multiplexing and de-multiplexing
between an optical line terminal (OLT) 102 and an optical network
unit (ONU) 104. The WDM filters 108 and 116 are connected by a
feeder fiber 101. The optical line terminal (OLT) 102 can be
coupled to a plurality of optical network units (ONUs) 104 via a
remote node 106. Each subscriber at an ONU 104 is assigned a
separate WDM channel, whereby the channels are routed by a passive
WDM filter 116 at the remote node 106. The OLT 102 includes a WDM
filter 108 coupled to a plurality of band separators 110-1 . . .
110-N. Each band separator 110-1 . . . 110-N is further coupled to
a transmitter 112-1, 112-2 . . . or 112-N and a receiver 114-1,
114-2 . . . or 114-N.
[0007] The passive WDM filter 116 at the remote node 106 provides a
plurality of channels that each channel terminates at one of the
ONU 104. Each ONU 104 includes a band separator 118-1 . . . 118-N
each coupled to a transmitter 120-1, 120-2 . . . or 120-N and a
receiver 122-1, 122-2 . . . or 122-N. The transmitters 120-1 . . .
120-N at the ONUs 104 provide narrow-spectrum light sources for
upstream signals in a band A. The transmitters 112-1 . . . 112-N at
the OLT 102 provide narrow-spectrum light source for the downstream
signals in a different wavelength band B. The transmitters 120-1 .
. . 120-N and 112-1 . . . 112-N are typically narrow-spectrum
distributed-feedback (DFB) laser diodes with various wavelength
tuning and stabilization mechanisms.
[0008] The above described conventional WDM-based optical network
includes several drawbacks. The wavelength-specific narrow-spectrum
light sources such as distributed-feedback (DFB) laser diodes are
complex to make and have limited range of wavelength tunability by
adjusting the laser operating temperature. The ONU for each
subscriber uses at least one such laser. A large number of the
wavelength-specific narrow-spectrum light sources are thus required
in the conventional WDM-based optical network 100, which typically
contains 32 or 40 ONUs. All the DFB laser chips have to be customer
designed to the specific wavelength range for each group of a few
wavelength channels. The emission center wavelengths of the DFB
laser sources typically need to be fine tuned individually by
temperature controller using precision spectral instrument to match
the ITU wavelength grid of each wavelength channel. The inventory
and field installation can become very complex and unmanageable in
large-scale deployment for access.
[0009] Furthermore, the shift in the narrow spectrum of a
wavelength-controlled DFB laser diode relative to the narrow
wavelength channels of the DWDM wavelength filter can significantly
affect the signal transmission. For example, a fraction of a degree
of temperature drift can drive the emission spectrum of a
narrow-spectrum laser out of the clear pass band window of the
wavelength channel and cause significant loss of transmission
signal. The reliability of the precision-temperature-controlled
narrow-spectrum lasers is therefore a challenge in WDM-PON
applications.
SUMMARY
[0010] In a general aspect, the present specification relates to an
optical module including a transmitter optical sub-assembly
comprising a transmitter configured to emit an
multi-longitudinal-mode (MLM) spectrum signal having an emission
spectrum comprising a plurality of distinct narrow-spectrum peaks
each corresponding to a longitudinal mode in the transmitter,
wherein the emission spectrum is configured to be shifted in
wavelength by a change in the transmitter temperature; a heating
and cooling device configured to control the temperature of the
transmitter in response to a temperature-control signal; and a
receiver optical sub-assembly configured to output a pair of
differential digital signals in response to an input optical
signal.
[0011] In another general aspect, the present specification relates
to an optical transceiver module including a transmitter configured
to emit a multi-longitudinal-mode (MLM) spectrum signal having an
emission spectrum comprising a plurality of distinct
narrow-spectrum peaks each corresponding to a longitudinal mode in
the transmitter, wherein the emission spectrum is configured to be
shifted in wavelength by changing the temperature of the
transmitter; a temperature sensor in thermal contact with the
transmitter, wherein the temperature sensor is configured to output
a temperature sensing signal in response to the temperature of the
transmitter; a temperature controller configured to control the
temperature of the transmitter in response to a temperature-control
signal; and a receiver configured to receive an input optical
signal and output a pair of differential digital signals, and
configured to output an analog monitoring signal in response to the
power of the input optical signal.
[0012] In yet another general aspect, the present specification
relates to an optical communication system including a) a plurality
of first optical transceiver modules each including a first
transmitter configured to emit a downstream multi-longitudinal-mode
(MLM) spectrum signal having an emission spectrum comprising a
plurality of distinct narrow-spectrum peaks each corresponding to a
longitudinal mode in the first transmitter, wherein the emission
spectrum of the downstream multi-longitudinal-mode (MLM) spectrum
signal is configured to be shifted in wavelength by changing the
temperature of the first transmitter; a first temperature
controller configured to control the temperature of the first
transmitter in response to a first temperature-control signal; and
a first receiver configured to output a pair of differential
digital signals in response to the upstream optical signal; and b)
a first wavelength filter including a plurality of first branching
ports each associated with a specific wavelength channel, wherein
each of the first branching ports is in connection with a first
optical transceiver modules and is configured to receive the
downstream MLM-spectrum signal from the first transmitter and send
an upstream signal to the first receiver; and a first common port
configured to output a downstream signal in response to the
downstream MLM-spectrum signal, wherein the spectrum of the
downstream signal is located in a wavelength channel specifically
associated with the first branching port at which the downstream
MLM-spectrum signal is received.
[0013] In yet another general aspect, the present specification
relates to an optical communication system including a) a first
optical transceiver module that includes: a first transmitter
configured to emit a downstream optical signal having a first
emission spectrum that is configured to be shifted in wavelength by
a change in the temperature of the first transmitter; a first
temperature controller configured to control the temperature of the
first transmitter in response to a first temperature-control
signal; and a first receiver configured to output a first digital
signal in response to an upstream optical signal; and b) a second
optical transceiver module that includes: a second transmitter
configured to emit the upstream optical signal having a second
emission spectrum that is configured to be shifted in wavelength by
a change in the temperature of the second transmitter; a second
temperature controller configured to control the temperature of the
second transmitter in response to a second temperature-control
signal; and a second receiver configured to output a second digital
signal in response to the downstream optical signal.
[0014] Implementations of the system may include one or more of the
following. The transmitter optical sub-assembly, the heating and
cooling device, and the receiver optical sub-assembly can be
integrated in a unitary device. The optical module can further
include a wavelength division multiplexing filter configured to
receive the input optical signal at an input/output port and send
the input optical signal to the receiver, and configured to receive
the MLM spectrum signal from the transmitter and output the MLM
spectrum signal at the input/output port. The optical module can be
a unitary device in which the transmitter optical sub-assembly, the
heating and cooling device, the wavelength division multiplexing
filter, and the receiver optical sub-assembly are integrated. The
optical module can further include a temperature sensor in thermal
contact with the transmitter, wherein the temperature sensor is
configured to output the temperature control signal to the heating
and cooling device in response to the temperature of the
transmitter. The heating and cooling device can be configured to
control the temperature of the transmitter in response to an
external signal. The receiver optical sub-assembly can be
configured to an analog monitoring signal in response to the power
of the input optical signal. The transmitter can be a Fabry-Perot
laser. The emission spectrum of the MLM spectrum source can be
characterized by a center wavelength, wherein the center wavelength
is configured to be shifted by more than 0.4 nm for a change of one
Celsius degree in the temperature of the transmitter.
[0015] Embodiments may include one or more of the following
advantages. The disclosed optical transceiver module allows an
optical communication system to include only passive devices
between the central office and the user's premises. As a result,
the complexity and maintenance associated with the disclosed
optical communication system can be significantly reduced comparing
to some conventional systems that use active devices in the field.
The use of passive devices in the fields also improves the system
reliability of the optical communication system.
[0016] The transmitter optical sub-assembly (TOSA) in the disclosed
transceiver module overcomes the drawbacks associated with the
wavelength-specific narrow-spectrum light sources in the
conventional systems. The disclosed optical communication system
uses temperature-stabilized multi-longitudinal mode (MLM) light
sources such as Fabry-Perot lasers as optical transmitters. The MLM
sources have much broader emission envelops than that of the
narrow-spectrum light sources (i.e. DFB lasers) in the conventional
DWDM based optical communication systems. The MLM light sources
also have wider wavelength tuning range with temperature comparing
to the narrow-spectrum DFB sources. The broad emission envelope and
a wide wavelength tunable range of the MLM light source allows the
same specification transmitters to be used for 32, 40, or even more
of the wavelength channels of a typical 100 GHz-spacing wavelength
filter, which eliminates the needs for maintaining a large
inventory of wavelength-specific transmitters.
[0017] The disclosed optical transceiver module based on an MLM
light source also exhibits robust performance. The active feedback
and control mechanism built into the transceiver module enable
reliable operations in the communication system. Small temperature
variations that can cause certain MLM modes to move out of the pass
band of a wavelength channel can be immediately detected by the
system and instantaneously compensated through the control
mechanism.
[0018] Furthermore, the optical transceiver module including a MLM
light source in the disclosed optical communication system can be
self-adaptive through built-in control capabilities. The broad
emission envelope of the MLM light sources in the disclosed optical
communication system can be shifted by adjusting the temperature of
the MLM light sources. Such temperature control and wavelength
tuning can be automatically carried out in the system interactively
or dynamically prior to or during the normal operation. The
transmitter having built-in self-adaptive feature is critical for
large scale deployment especially with vast number of ONU's in the
field. The disclosed transceiver module can provide real-time
feedback about the status of the communication channel, to improve
the performance of the optical communication system. The disclosed
system based on MLM sources can achieve high speed of data
transmission under outdoor uncontrolled environment. For example,
the disclosed optical communication system can achieve data rate of
several Gigabits per second (Gbps) per ONU, which is an order of
magnitude higher than other TDM-based PON systems. The disclosed
system can provide bandwidth capacity, system stability, and
robustness unmatched by conventional WDM-PON systems based on other
types of transmitter configurations, for example, injection-locked
laser or reflective semiconductor optical amplifier.
[0019] The receiver optical sub-assembly (ROSA) in the disclosed
optical transceiver module can be implemented with dual
functionalities of digital signal detect and optical channel
monitor. Such implementation removes the requirements for
additional optical tap monitor specifically for power monitoring
purpose, which could significantly reduce the system cost.
[0020] Another advantage of the disclosed optical communication
system is that it provides flexibility for network configuration,
integration, and management. The disclosed optical communication
system is agnostic to different communication protocols. Unlike
conventional TDM-PON systems that need extra protocols (TDMA,
RANGING) between optical layer and data layer, the signal
transmission between the OLT and ONU in the disclosed optical
communication system operates in a continuous mode and each ONU
occupies a dedicated channel. The system can naturally adapt to any
communication protocols at any bit-rate.
[0021] Yet another advantage of the disclosed optical communication
system is that each ONU can communicate in an independent channel.
The bandwidth capacity for each ONU can be upgraded without
changing the overall optical communication system and at minimal
incremental cost, which greatly extends the lifetime of the
installed devices and components. In contrast, the downstream and
upstream bandwidths are shared by all users in a conventional PON
(or TDM-PON) system. Any bandwidth increase for one user will
affect the resource allocation and the operation of the entire
system. The conventional PON (or TDM-PON) is thus not scalable and
extremely bit-rate and protocol dependent. Thus the disclosed
optical communication system can provide much improved bandwidth
scalability, upgrade flexibility and performance robustness.
[0022] Each ONU in the disclosed system occupies a unique
wavelength channel. The channel spacing can be anywhere from a few
hundred picometers (in the case of DWDM) to tens of nanometers (in
the case of CWDM). Dispersion and optical non-linear effects
usually have less impact on signal quality because of the short
distance in the access applications. Because of the cyclic
characteristic of the wavelength filter (AWG), hundreds of
wavelength channels can potentially be used for network expansion.
In addition, each wavelength channel can operate independently in
continuous mode. The bandwidth for each ONU can be upgraded from
100 Mbps to 1 Gbps, 2.5 Gbps, or even higher. The total throughput
of one WDM-PON can be as high as 40.about.100 Gbps, which provides
much needed bandwidth for future expansions.
[0023] The disclosed optical communication system includes a number
of other advantages. The disclosed optical communication system can
provide symmetrical bandwidths for downstream and upstream signals.
The bandwidth symmetry allows high bit-rate data transfers both
downstream and upstream directions, which is a significant
improvement over TDM-PONs (APON, BPON, EPON and GPON) and the
conventional systems based on DSL and cable modems. The disclosed
optical communication system also provides excellent network
security and communication privacy because each ONU occupies a
distinct wavelength channel and is physically isolated from other
wavelength channels at optical layer.
[0024] Although the specification has been particularly shown and
described with reference to multiple embodiments, it will be
understood by persons skilled in the relevant art that various
changes in form and details can be made therein without departing
from the spirit and scope of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The following drawings, which are incorporated in and from a
part of the specification, illustrate embodiments of the present
specification and, together with the description, serve to explain
the principles of the specification.
[0026] FIG. 1 is a block diagram of a conventional WDM-based
optical network.
[0027] FIG. 2A is a block diagram of an optical communication
system using MLM sources as transmitters in accordance to an
embodiment of the present specification.
[0028] FIG. 2B is a detailed view of the wavelength filter in the
optical line terminal in the optical communication system of FIG.
2A.
[0029] FIG. 2C is a detailed view of the wavelength filter at the
remote node in the optical communication system of FIG. 2A.
[0030] FIG. 3A and 3B respectively illustrate exemplified
implementations of the MLM transmitters in an OLT and an ONU.
[0031] FIG. 4A illustrates the emission spectrum of an MLM source
in accordance to the present specification.
[0032] FIG. 4B is an expanded view of the emission spectrum of an
MLM source illustrating the spectral profiles of individual modes
and the spacing between the modes.
[0033] FIG. 5A illustrates the spectral distribution of the
wavelength channels of the wavelength filters.
[0034] FIG. 5B illustrates the spectrum of a MLM source at two
different temperatures T.sub.1 and T.sub.2.
[0035] FIG. 5C illustrates the temperature dependence of the center
wavelength of a typical MLM source.
[0036] FIG. 6 is a block diagram of an integrated bi-directional
optical sub-assembly (OSA).
[0037] FIG. 7A is a block diagram of a receiver optical
sub-assembly (ROSA).
[0038] FIG. 7B is a block diagram of a receiver module with a
relative signal strength indicator (RSSI) output.
[0039] FIG. 8A is a block diagrams for an implementation of a
temperature-controlled transmitter optical sub-assembly.
[0040] FIG. 8B illustrates an exemplary arrangement of the key
components in the temperature-controlled transmitter optical
sub-assembly of FIG. 8A.
[0041] FIG. 8C is a cross sectional view of a Fabry-Perot laser
cavity.
[0042] FIG. 9A illustrates the construction of a conventional
bi-directional optical sub-assembly in a low cost transistor
outline can (TO-CAN) package.
[0043] FIG. 9B illustrates a bi-directional optical sub-assembly
having a tunable MLM TOSA in accordance with an embodiment of the
present specification.
[0044] FIG. 10 is a block diagram of a transceiver module in
accordance with an embodiment of the present specification.
DETAILED DESCRIPTION
[0045] FIG. 2A shows an optical communication system 200
in-accordance with an embodiment of the present specification. The
optical communication system 200 includes an OLT 202, a remote node
(RN) 204 in connection with the OLT 202 through an optical network,
and a plurality of ONUs 206-1 . . . 206N in connection with the RN
204.
[0046] The optical communication system 200 includes two symmetric
wavelength filters: a wavelength filter 212 in the OLT 202 and a
wavelength filter 222 at the RN 204. The wavelength filter 212 and
the wavelength filter 222 are wavelength division multiplexing
(WDM) filters. The wavelength filters 212 and 222 can be
implemented by arrayed-waveguide gratings (AWG) that can be tuned
to the common communication bands, including O, E, S, C, L or
U-band and typically follow the wavelength grids of International
Telecommunication Union (ITU). The wavelength filters 212 or 222
can also be based on other forms of WDM filters such as thin-film
DWDM and CWDM filters.
[0047] The wavelength filter 212 or 222 can receive MLM source
signals at separate branching ports (i.e. 212b1, 212b2 . . . 212bN
and 222b1, 222b2 . . . 222bN as shown FIGS. 2B and 2C) as inputs
and filter (or slice) the MLM source signals to output multiplexed
spectrum-sliced signals at the common ports (i.e. 212c, and 222c in
FIGS. 2B and 2C) of the wavelength filter 212 or 222. Each of the
spectrum-sliced signals carries data from the respective input MLM
source signals. The output spectrum-sliced signals are respectively
located in a plurality of predetermined wavelength channels "Ch1",
"Ch2" . . . "Ch N" identical to both wavelength filters 212 and
222. The wavelength channels "Ch1", "Ch2" . . . "Ch N" are
determined by the pass bands of the wavelength filters 212 and 222,
and characterized by the unique channel center wavelengths
(.lamda..sub.Ch1, .lamda..sub.Ch2 . . . .lamda..sub.ChN), the pass
band width and the optical isolation between each wavelength
channel. The adjacent channel spacing
(|.lamda..sub.Chi.lamda..sub.Chi|, i=2, 3 . . . N) between the
wavelength channels "Ch1", "Ch2" . . . "Ch N" of the filters 212 or
222 can range from hundreds of picometer to tens of nanometer.
[0048] A detailed view of the wavelength filter 212 in the OLT 202
is shown in FIG. 2B. The wavelength filter 212 includes a plurality
of branching ports 212b1, 212b2 . . . and 212bN, and a common port
212c. Each of the branching ports 212b1, 212b2 . . . or 212bN is
associated with a distinct and specific wavelength channel "Ch1",
"Ch2" . . . or "Ch N". The wavelength filter 212 can receive a
downstream MLM source signal at a branching ports 212b1, 212b2 . .
. or 212bN, and filter (or slice) the spectrum of the downstream
MLM source signal. The wavelength filter 212 then outputs a
downstream spectrum-sliced signal at the common port 212c. The
spectrum of the downstream spectrum-sliced signal is located in the
specific wavelength channel associated with the branching port
212b1, 212b2 . . . or 212bN at which the downstream MLM source
signal is received. In other words, the spectrum of the downstream
spectrum-sliced signal output at the common port 212c is determined
by the wavelength channel associated with the branching port 212b1,
212b2 . . . or 212bN at which the input downstream broad-spectrum
signal is received.
[0049] The wavelength filter 212 can also process optical signals
in the reverse direction. An upstream spectrum-sliced signal
(received from the wavelength filter 222 via the feeder fiber 218
and the optional optical amplifier 216) can be received at the
common port 212c. The upstream spectrum-sliced signal is
characterized by a spectrum in a specific wavelength channel "Ch1"
or "Ch2" . . . "Ch N". The wavelength filter 212 can route the
upstream spectrum-sliced signal to one of the branching ports
212b1, 212b2 . . . or 212bN in accordance with the wavelength
channel of the upstream spectrum-sliced signal. The routing is so
arranged that the wavelength channel of the upstream
spectrum-sliced signal matches the wavelength channel of the
receiving branching port 212b1, 212b2 . . . or 212bN. The upstream
spectrum-sliced signal routed to a branching port 212b1, 212b2 . .
. or 212bN is subsequently transmitted to one of the transceiver
ports 209-1, 209-2, or 209-N.
[0050] The central wavelength of wavelength filters can be
sensitive to temperature variations. In one implementation, the
wavelength filters 212 or 222 can be based on a thermal AWGs, which
become commercially available recently. The a thermal AWGs can
include various temperature compensation mechanisms to reduce the
sensitiveness of the AWG-based wavelength filters 212 and 222 to
temperature variations and to allow them be installed in an
environment without temperature control. This capability of the
disclosed optical communication system can significantly reduce the
complexity and cost for field installations.
[0051] The optical communication system 200 further includes a
plurality of transceiver ports 209-1, 209-2 . . . 209-N that can
reside in the OLT 202. Each transceiver port 209-1, 209-2 . . .
209-N can include a transmitter 208-1 (or 208-2 . . . 208-N) for
providing MLM downstream optical signals and a receiver 210-1 (or
210-2 . . . 210-N) for receiving upstream optical signals.
Connecting the transmitter and the receiver is a signal
separating/combining device 214-1 (or 214-2 . . . 214-N).
[0052] In one embodiment, the transceiver port 209-1, 209-2 . . .
209-N can be based on the various implementations of the integrated
optical transceiver modules as disclosed below in FIGS. 6-10.
Specifically, the transceiver port 209-1, 209-2 . . . 209-N can be
bi-directional integrated optical transceiver modules that can
receive upstream signals and output down steam MLM-source signals
at a single optical connector. The integrated optical transceiver
modules can include temperature control and sensing capabilities
for the MLM-source transmitters 208-1 . . . 208-N. The integrated
optical transceiver modules can also provide output signals that
represent the power levels of the MLM-source transmitters 208-1 . .
. 208-N.
[0053] Each transceiver port 209-1, 209-2, . . . 209-N is connected
with one of the branching ports 212b1, 212b2 . . . 212bN of the
wavelength filter 212 and is thus associated with a specific
wavelength channel "Ch1", "Ch2" . . . "Ch N" of the wavelength
filter 212. The wavelength filter 212 can be coupled with the
transceiver ports 209-1, 209-2 . . . 209-N by single-mode optical
fibers. The MLM signals produced by the transmitters 208-1, 208-2,
. . . 208-N are sliced by the wavelength filter 212 to produce
multiplexed spectrum-sliced signals each occupying a wavelength
channel specific to the respective branching port 212b1, 212b2 . .
. or 212bN of filter 212. The receivers 210-1, 210-2 . . . 210-N
are configured to receive spectrum-sliced signals having their
wavelength channels specific to the respective branching ports
212b1, 212b2 . . . and 212bN of the wavelength filter 212.
[0054] The optical system 200 has a symmetrical architecture, which
also includes a plurality of transceiver ports 206-1, 206-2 . . .
206-N in each ONU distributed in the field. Each transceiver port
206-1, 206-2 . . . 206-N contains a transmitter 228-1 (or 228-2 . .
. 228-N) for providing MLM upstream optical signal and a receiver
220-1 (or 220-2 . . . 220-N) for receiving MLM downstream optical
signals. Connecting the transmitter and the receiver is a signal
separating/combining device 224-1 (or 224-2 . . . 224-N).
[0055] In the present specification, the term "downstream signal"
refers to an optical signal sent from service provider's central
office to users' premises. The term "upstream signal" refers to an
optical signal sent from the users' premises to a central office.
The term "MLM source" or "multi-longitudinal mode source" refers to
an optical signal that has a spectrum with composite of peaks
(modes) wherein the envelope joining the modal peaks having a
full-width at half the maximum (FWHM) equal to or greater than 1
nanometer. A "narrow spectrum" refers to an optical signal that has
a spectral FWHM of the line profile less than 1 nanometer and also
its side modes are suppressed by a minimum of 10 dB. A
spectrum-sliced signal refers to the signal sliced (or filtered)
from a "MLM source" unless it is otherwise specified. Thus the
spectral FWHM of a spectrum-sliced signal is a fraction of the
spectral envelope FWRM of the original "MLM source" signal.
[0056] The transmitters 208-1, 208-2 . . . 208-N and 228-1, 228-2 .
. . 228-N can be based on MLM sources that can be directly
modulated to carry the downstream optical signals. One example for
the MLM source transmitter is multi-longitudinal mode Fabry-Perot
lasers. The transmitters 208-1, 208-2 . . . 208-N and 228-1, 228-2
. . . 228-N can also be implemented by temperature controlled super
luminescent diodes (SLD) and its variant. Fabry-Perot lasers are
less costly and much easier to maintain compared to the
wavelength-specific narrow-spectrum transmitters (such as DFB
lasers) in the conventional optical systems. The MLM transmitters
208-1, 208-2 . . . 208-N and 228-1, 228-2 . . . 228-N, the
receivers 210-1 . . . 210-N and 220-1, 220-2 . . . 220-N and the
signal separating/combining devices 214-1,214-2 . . . 214-N and
224-1, 224-2 . . . 224-N can be integrated to a unitary device for
bi-directional signal transmission (discussed below in FIG. 6-8),
which can reduce form-factor and costs.
[0057] The transmitters 208-1 . . . 208-N and 228-1 . . . 228-N can
be modulated at rates ranging from hundreds to thousands of megabit
per second (Mbps) modulation speed. The transmitters 208-1 . . .
208-N and 228-1 . . . 228-N can provide stable MLM light sources
with minimal or no instabilities caused by external optical
feedback or back-reflection. In certain applications, special
measures may be required to reduce any instability that might be
induced by reflection or backscattering. The center wavelengths
(CW) of the common MLM source signals can be designed anywhere in
the optical spectrum of the communication window for the common
optical fibers, which can be from 1100 nm to 1700 nm.
[0058] An advantage of the use of MLM source in the optical
communication system 200 is that the transmitter 208-1 . . . 208-N
or 228-1 . . . 228-N can be easily tuned and locked at a specified
center wavelength. The optical communication system 200 can cover a
large number of individual wavelength channels. The center
wavelength of each MLM source can be stabilized by a temperature
controller. As shown in FIG. 3A, the transmitter 208-1 in the OLT
202 can include a multi-longitudinal mode source (MLM) 250 and a
temperature controller 251. MLM 250 is in thermal contact with the
temperature controller 251. The temperature controller 251 can be
thermal electric temperature controller built-in the MLM source
208-1.
[0059] The broad envelope of the emission spectrum combining the
fine pith of mode-spacing of the MLM source could relax the
temperature control requirements for MLM source comparing to those
of the DFB lasers. The DFB lasers typically require temperature
control to achieve wavelength accuracy within 0.1 nanometer and to
guard against long-term aging of the laser and the temperature
control system. The MLM source in the disclosed system can be more
tolerant. In some implementations, the MLM sources suitable for the
transmitters 208-1 . . . 208-N and the transmitters 228-1 . . .
228-N can accept wavelength accuracy >0.1 nanometer and the
transceiver system could have the capabilities to correct transient
or aging related drifts with the built-in feedback/control systems
described in details below. The temperature controller 251 (and
261) can be implemented by standard, low-cost controller devices.
As discussed in more detail below, the wavelength tuning and
control of the MLM sources can be fully automatic. The emission
spectra for transmitters 208-1 . . . 208-N and the transmitter
228-1 . . . 228-N can be controlled by simply setting the control
temperature to their corresponding set points, which could be
sufficient to cover all the wavelength channels of the wavelength
filters 212 and 222.
[0060] In one embodiment, the ONUs 206-1 . . . 206-N can be based
on the various implementations of the integrated optical
transceiver modules as disclosed below in FIGS. 6-10. Specifically,
the ONUs 206-1 . . . 206-N can be bi-directional integrated optical
transceiver modules that include temperature control and sensing
capabilities for the MLM-source transmitters 228-1 . . . 228-N. The
integrated optical transceiver modules can also provide output
signals that represent the power levels of the MLM-source
transmitters 228-1 . . . 228-N.
[0061] The wavelength filter 212 can receive the MLM source optical
signals produced by the transmitter 208-1 . . . 208-N and filter
(or slice) the MLM source optical signals to produce multiplexed
spectrum-sliced optical signals at the common port 212c. The
spectrum of each MLM source optical signal is tuned specifically to
be associated with the branching port 212b1, 212b2 . . . and 212bN
of the wavelength filter Ch1 . . . ChN to which the MLM source
signal is transmitted.
[0062] The wavelength filters 212 and 222 based on AWGs can be
cyclic over a wavelength range. The pass band for a
spectrum-slicing channel (Ch1, Ch2 . . . and ChN) can be cyclic in
the optical spectrum. Each channel (Ch1, Ch2 . . . ) can have
multiple pass-band peaks separated by a free spectral range (FSR).
The periodicity or free spectral range can be varied by design.
Furthermore, the FSR may be designed to be close to the overall AWG
pass band width (defined by the wavelength span between the center
wavelengths of the first and the last filter channel within the
same FSR: |.lamda..sub.ChN.lamda..sub.Ch1|).
[0063] One advantage of the AWG based filters 212 or 222 is that
the downstream and upstream traffics can be separated by a
wavelength of one or more FSRs for each channel ("Ch1", "Ch2" . . .
"Ch N"). For example, a bidirectional system can be implemented
such that the downstream signals occupy a sequence of center
wavelengths--.lamda..sub.Ch1, .lamda..sub.Ch2 . . . .lamda..sub.ChN
in C band while upstream signals occupy a sequence of center
wavelengths--(.lamda..sub.Ch1+n.times.FSR),
(.lamda..sub.Ch2+n.times.FSR) . . . (.lamda..sub.ChN+n.times.FSR),
where n=0 or .+-.1 or .+-.2 . . . possibly in a different band.
[0064] Each transceiver port 209-1 . . . 209-N can include a signal
separating/combining device 214-1 . . . 214-N to assist
bi-directional communications in either downstream or upstream
directions. These signal separating/combining devices 214-1 . . .
214-N can be implemented by WDM filters, power splitter, and
circulators. The signal separating/combining devices 214-1 . . .
214-N are respectively coupled with the transmitter 208-1 . . .
208-N and the receivers 210-1 . . . 210-N in the respective
transceiver ports 209-1 . . . 209-N. The signal
separating/combining devices 214-1 . . . 214-N are also coupled
with the wavelength filters 212, each of which can include a single
optical fiber connection. In the implementation of WDM filters
combining with AWG as wavelength filter, the signal
separating/combining devices 214-1 . . . 214-N can use filter
function to separate signal in different regions of the FSR for the
downstream optical signals from the transmitter 208-1 . . . 208-N
and the upstream optical signals to be received by the receivers
210-1 . . . 210-N. Thus the signal separating/combining devices
214-1 . . . 214-N can enable bi-directional transmission of optical
signals with a single optical connection to the wavelength filter
212. The temperature-controlled MLM source 208-1,208-2 . . . 208-N,
the receiver photodiode 210-1,210-2 . . . 210-N and the WDM filter
based signal separating/combining device 214-1,214-2 . . . 214-N
can be integrated into a unitary bi-directional optical
sub-assembly (OSA), which is to be discussed in detail in
connection with FIGS. 6-10.
[0065] The wavelength filter 222, typically mirroring that of the
filter 212 in optical specifications, is optically connected with
the plurality of ONUs 206-1 . . . 206-N. Each of the ONUs 206-1 . .
. 206-N is specifically associated with a counterpart transceiver
port 209-1 . . . 209-N in the OLT 202 and is characterized by a
specific wavelength channel determined by the filter function of
the filters 212 and 222. Each wavelength channel can carry
bidirectional signals.
[0066] A detailed view of the wavelength filter 222 in the RN 204
is shown in FIG. 2C. The wavelength filter 222 includes a plurality
of branching ports 222b1, 222b2 . . . and 222bN, and a common port
222c. Each of the branching ports 222b1, 222b2 . . . and 222bN is
associated with a distinct and specific wavelength channel "Ch1",
"Ch2" . . . or "Ch N". Each branching port 222b1, 222b2 . . . or
222bN is respectively connected with an ONU 206-1 . . . 206-N. The
wavelength filter 222 can receive an upstream MLM signal at a
branching ports 222b1, 222b2 . . . or 222bN from an ONU 206-1 . . .
206-N, and filter (or slice) the spectrum of the upstream MLM
signal. The wavelength filter 222 then outputs an upstream
spectrum-sliced signal at the common port 222c (via feeder fiber
218). The spectrum of the upstream spectrum-sliced signal is
located in the specific wavelength channel associated with the
branching port 222b1, 222b2 . . . or 222bN at which the upstream
broad-spectrum signal is received. In other words, the spectrum of
the upstream spectrum-sliced signal output at the common port 222c
is determined by the wavelength channel associated with the
branching port 222b1, 222b2 . . . or 222bN at which the input
upstream MLM signal is received.
[0067] Each ONU 206-1 . . . 206-N can include a transmitter 228-1
(or 228-2 . . . 228-N) for providing a MLM upstream optical signals
and a receiver 220-1 (or 220-2 . . . 220-N) for receiving
downstream optical signals and a signal separating/combining device
224-1 (or 224-2 . . . 224-N). Each ONU 206-1, 206-2 . . . 206-N is
connected with a branching port 222b1, 222b2 . . . 222bN of the
wavelength filter 222 and is associated with a specific wavelength
channel "Ch1", "Ch2" . . . "Ch N" of the wavelength filter 222. The
wavelength filter 222 can be coupled with the ONUs 206-1 . . .
206-N by single-mode optical fibers. The MLM signals produced by
the transmitters 228-1 . . . 228-N are sliced by the wavelength
filter 222 to produce multiplexed upstream signals with specific
wavelength channels determined by the branching ports 222b1, 222b2
. . . and 222bN of the wavelength filter 222.
[0068] The wavelength filter 222 can receive downstream
spectrum-sliced signal via the feeder fiber 218 at the common port
222c. The downstream spectrum-sliced signal is characterized by a
wavelength channel of one of the branching ports 212b 1, 212b2 . .
. and 212bN of the wavelength filter 212. The wavelength filter 222
can route the downstream spectrum-sliced signal to one of the
branching ports 222b1, 222b2 . . . or 222bN in accordance with the
wavelength channel of the downstream spectrum-sliced signal such
that the wavelength channel of the downstream spectrum-sliced
signal matches the wavelength channel of the receiving branching
port 222b1, 222b2 . . . or 222bN. The downstream spectrum-sliced
signal routed to a branching port 222b1, 222b2 . . . or 222bN is
subsequently transmitted to one of the ONUs 206-1 . . . 206-N.
[0069] The receivers 220-1 . . . 220-N in the ONUs 206-1 . . .
206-N are configured to receive downstream signals that are
transmitted through the specific filter channel. As an example, the
ONU 206-1 and the OLT 209-1 share the same wavelength
channel--"Ch1". The ONU 206-2 and the transceiver port 209-2 share
the same wavelength channel "Ch2", and so on. Each ONU 206-1 . . .
206-N includes a signal separating/combining device 224-1 (or 224-2
. . . 224-N), a transmitter 228-1 (or 228-2 . . . 228-N), and a
receiver 220-1 (or 220-2 . . . 220-N). The transmitters 228-1 . . .
228-N can be MLM sources, which may have different implementations
from the transmitter 208-1 . . . 208-N. FIG. 3B shows an
exemplified implementation of the transmitter 228-1 at the ONU
206-1. The transmitter 218-1 includes a MLM source 260 and a
temperature controller 261 that can control the temperature of the
MLM source 260. The temperature controller 261 can be a thermal
electric temperature controller that is built-in the MLM
transmitter 228.
[0070] It should be noted that although an ONUs 206-1 . . . 206-N
and its counterpart transceiver port 209-1 . . . 209-N in the OLT
202 share the communication tasks in each channel "Ch1", "Ch2" . .
. or "ChN", they do not have to operate in exactly the same
wavelength range for both downstream and upstream transmission. For
example, utilizing the cyclic features in the case of AWGs as the
wavelength filters 212 and 222, the downstream and upstream signals
can occupy different wavelengths, which are separated by a multiple
of FSRs.
[0071] The transmitter 228-1 . . . 228-N can produce MLM upstream
signals to be sent to the common port 222c at the wavelength filter
222 wherein the MLM upstream signals are sliced (or filtered) into
specific wavelength channels. For example, the MLM upstream signal
from the ONU 206-1 is filtered by the wavelength filter 222 to
produce a spectrum-sliced upstream signal in the wavelength channel
"Ch 1" that is also specific to the transceiver port 209-1. The
spectrum-sliced upstream signal can be amplified if necessary,
passing through the wavelength filter 212 and the signal
separating/combining device 214-1, and being received by the
receiver 210-1 in the transceiver port 209-1.
[0072] In the downstream direction, the MLM optical signal produced
by the transmitter 208-1 passes the signal separating/combining
device 214-1 and is sliced (or filtered) by the wavelength filter
212 into a spectrum-sliced downstream signal in the wavelength
channel "Ch 1". The spectrum-sliced downstream signal is next
amplified if necessary and transmitted to the wavelength filter 222
at the RN 204. The wavelength filter 222 then routes the
spectrum-sliced downstream signal in "Ch 1" to the ONU 206-1 that
is characterized by the same wavelength channel "Ch 1". As
described, each of the ONUs communicates downstream or upstream in
its specific wavelength channel within each system. The secure
wavelength specific communications in the disclosed system is a
significant improvement over the broadcasting mode of
communications in some conventional systems.
[0073] FIG. 4A illustrates the emission spectrum 400 of a typical
MLM source. FIG. 4B is an expanded view of the emission spectrum of
a typical MLM source. The emission spectrum 400 includes a
plurality of individual emission modes 401. The envelope 405 of the
individual emission modes 401 is formed by joining all the peaks of
the modes 401, and only serves as a visual guide. In the present
specification, the center wavelength (CW) 403 of the MLM emission
spectrum can be defined by the power-weighed average of the peaks
of the individual modes 401:
CW.sub.MLM=.SIGMA.(p.sub.i*.lamda..sub.i)/.SIGMA.(p.sub.i) where
p.sub.i and .lamda..sub.i are the power (in linear units) and the
center wavelength of individual modes respectively. The summation
covers over all the peaks within 20 dB range of the strongest peak.
In accordance with one aspect of the present specification, the
side-modes in the MLM sources are not suppressed; instead the side
modes are used to achieve desirable spectrum-slicing effects by the
wavelength filter.
[0074] An effective width 404 of the MLM emission spectrum can be
defined by the spectral FWHM of the envelope 405. The spectral
width of envelope 405 of the MLM emission spectrum can commonly be
represented by the full width at specific "x" decibel value (dB)
below the maximum (PW.times.dB). A common specification of the
spectral width is the full width at half the maximum (FWHM), which
is equivalently to PW3 dB. Analogously, the pass bands 415 of the
wavelength channel of a wavelength filter can be defined in the
same fashion and labeled as BW.times.dB. Each modal peak has a
spectral line width 407. The spacing 406 between adjacent
individual modes is defined by the wavelength difference between
the neighboring peaks .lamda..sub.i+1-.lamda..sub.i. The line width
(FWHM) 407 of an individual modal peak is typically much narrower
than 1 nm. The mode spacing 406 is not restricted in the current
specification if part or all of the subsequent controls are
implemented.
[0075] In accordance to the present specification, the emission
spectrum of a MLM source can be tuned like a tunable laser to cover
part or all the wavelength channels of the wavelength filters 212
and 222. FIG. 5A illustrates the spectral distribution of the
wavelength channels of the wavelength filters (e.g. 212, 222,) at
center wavelengths .lamda..sub.1 .lamda..sub.2 . . . .lamda..sub.N.
FIG. 5B illustrates the spectrum of a MLM source at two different
temperatures T.sub.1 and T.sub.2. FIG. 5C illustrates the
temperature dependence of the center wavelength of a typical MLM
source. MLM sources such as the Fabry-Perot lasers have large
temperature sensitivity, which allows the center wavelength of the
emission spectrum of the MLM source to be tuned with relatively
small variation of the temperature. For example, the temperature
sensitivity of the emission wavelength of a Fabry-Perot laser can
be more than 0.4 nm/.degree. C. It is therefore possible to use
transmitters based on the same Fabry-Perot laser to cover a large
number (e.g. 32, 40 and 48) of the wavelength channels in a given
optical communication system 200. A 100 GHz spacing wavelength
filter will occupy .about.25 nm of spectral range for 32 channels
in the C-band, A 50 degree of temperature tuning can cover the same
spectral range for a temperature sensitivity factor of 0.5
nm/.degree. C. The temperature controllers 251 and 261 as shown in
FIGS. 3A and 3B can be controlled to set the MLM sources to
different temperature set-points such that the respective
transmitters can provide stable MLM source signal for wavelength
channels in different wavelength ranges. It should be noted that
the thermal tuning of the center wavelength of an emission spectrum
is applicable to other optical sources such as LED and SLD
sources.
[0076] An important feature of the optical communication system 200
is that the transmitters 208-1, 208-2 . . . 208-N and transmitters
228-1, 228-2 . . . 228-N are adaptive to the spectral pass bands of
the wavelength channels "Ch1", Ch2" . . . "ChN". In the present
specification, the spectral adaptability to the wavelength channels
by the transmitters is achieved by automatic tuning of the
temperature of each transmitter in the system. The controlled
temperature change of a MLM light source (i.e. Fabry-Perot laser)
can cause a shift in the center wavelength of the emission spectrum
such that one strong mode of the MLM source aligning with the
particular wavelength channel. The spectral shift can also be
monitored by measuring the optical output power at the
corresponding receiving side of the system. For example, as the
temperature of the transmitter 228-1 is controlled to change, the
center wavelength of emission spectrum of the transmitter 228-1
will shift relative to the pass band of the wavelength channel
"Ch-1". The optical power of the upstream spectrum-sliced signal
detected at the corresponding receiver 210-1 will vary based on the
relative spectral positions of the emission spectrum and the pass
band of the channel. This information of power variation of the
upstream spectrum-sliced signal detected 210-1 can be sent
downstream by the transmitter 208-1 to be used as a feedback to
control/adjust the temperature setting of the transmitter 228-1.
Conversely, as the temperature of the transmitter 208-1 is
controlled to change, the center wavelength of emission spectrum of
the transmitter 208-1 will shift relative to the pass band of the
wavelength channel "Ch-1". The optical power of the downstream
spectrum-sliced signal detected at the corresponding receiver 220-1
will vary based on the relative spectral positions of the emission
spectrum and the pass band of the channel. This information of
power variation of the downstream spectrum-sliced signal detected
220-1 can be sent upstream by the transmitter 228-1 to be used as a
feedback to control/adjust the temperature setting of the
transmitter 208-1.
[0077] FIG. 6 is a block diagram of a bi-directional optical
sub-assembly (OSA) 600. A MLM transmitter optical sub-assembly
(TOSA) 800, a receiver optical sub-assembly (ROSA) 700, and a
signal separating/combining WDM filter 601 are integrated into the
unitary bi-directional OSA 600. In some implementations, an optical
lens or collimator 603 can be provided to efficiently couple
input/output lights at a common input/output port. The ROSA 700 is
aligned precisely to receive the incoming optical signal deflected
by the WDM filter 601 at a specific angle, for example 45.degree..
An implementation of the ROSA 700 is illustrated in FIG. 7A. The
most commonly used WDM filter 601 is a thin-film filter that is
designed such that specific wavelength optical signal can be
reflected with little loss and be intercept by the ROSA 700. The
MLM source signal produced by the TOSA 800 is intercepted by the
WDM filter 601 also at a predetermined angle. The WDM filter 601
can pass the MLM source signals within a certain wavelength range
with very little loss. Then the MLM source signal can be coupled
into an external optical fiber at the input/output port. The MLM
source signals propagating in the optical fiber can be guided to
the branching ports of the wavelength filters 212 and 222 in the
optical communication system 200, as described above. The
integrated bi-directional optical sub-assembly 600 can therefore be
a unitary device in the place of the transceiver ports 209-1,
209-2, or 209-N and the ONUs 206-1, 206-2 . . . 206N. A feature of
the integrated bi-directional optical sub-assembly 600 is that the
input optical signals and the output MLM source signals can share
the same input/output port in a unitary device.
[0078] A receiver optical sub-assembly (ROSA) often includes a
signal-detect (SD) output. The signal detect is commonly
implemented by a simple level comparator. In the present
specification, the signal-detect signal can be used to provide a
coarse feedback for the spectral alignment between a wavelength
channel and the central wavelength of a MLM transmitter. When
optical power is larger than a pre-determined level, the SD signal
is asserted, otherwise, SD signal will be de-asserted. The
assertion and de-assertion of the signal-detect signal indicate an
alignment window of the MLM source relative to the pass band of a
corresponding wavelength channel. The signal-detect signal can be
used as a control signal to set the temperature of the MLM source.
The SD signal is a binary output that only indicates two stages
(good or bad) of wavelength alignment. Sometimes fine tuning
capability is needed in the optical communication systems 200 in
order to find the optimal position within the alignment window,
which will improve the link budget and reliability. In these
situations, a more accurate power level indicator such as an analog
power monitor can be implemented in the ROSA.
[0079] FIG. 7A illustrates an ROSA 710 compatible with the ROSA 700
in the integrated bi-directional optical transceiver module 600.
The ROSA 710 includes a photo diode 702 and a transimpedance
amplifier (TIA) 706. The photo diode 702 is in connected with the
transimpedance amplifier 706. The transimpedance amplifier 706 can
convert photo-current signal received from the photo diode 702 to
two differential voltage output signals "Data+" and "Data-". The
transimpedance amplifier 706 includes an additional lead 716 that
can simultaneously output an analog signal that is largely
proportional to the photo-current of the photo diode 702, which can
be used as an indicator of the optical power of the input optical
signal. A beneficial feature of ROSA 710 is that the output analog
photo-current signal can be conveniently used as a feedback signal
for tuning temperature controlled transmitters (implemented at the
place of transmitters 208-1 . . . 208-N and 228-1 . . . 228-N) in
the optical communication system 200. TIA chips 706 including
monitor functions are commercially available. For example,
conventional ROSA based on transistor outline can package (TO-CAN)
has 4 output pins. A 5-pin TO-CAN ROSA can be implemented to allow
the monitor signal to be directly wire-bonded from a TIA chip.
[0080] FIG. 7B shows a receiver 715 including power monitoring
capability. A conventional 4-pin ROSA 720 includes a photo diode
712 and a transimpedance amplifier (TIA) 716. The differential data
output signals "Data+" and "Data-" of the transimpedance amplifier
716 are respectively connected with two inputs of a post amplifier
722. The post amplifier 722 outputs a signal-detect signal and a
RSSI (Relative Signal Strength Indicator) signal. The RSSI signal
is an analog signal that is largely proportional to the amplitude
of the differential data signals "Data+" and "Data-". An AGC
(Automatic Gain Control) loop is commonly implemented in the
transimpedance amplifier 716, which disproportions the differential
output signals "Data+" and "Data-". For example, when optical input
power is very small, the transimpedance of the TIA 716 is very
large and the differential output signals are sensitive to optical
power variations. When optical input power increases, the AGC loop
will reduce the transimpedance of TIA 716. As a result, the
amplitude of differential output signals "Data+" and "Data-" has a
non-linear relation with the optical input power. Thus, as an
optical power strength indicator, RSSI has a large dynamic range
but a poor linearity.
[0081] FIG. 8A is a block diagrams for a temperature-controlled
TOSA 800. The TOSA 800 contains a MLM source 801, a built-in
temperature sensor 802, a heating and cooling device (H/C) 805, and
a back-facet photo photodiode monitor 803. FIG. 8B illustrates a
typical arrangement of each key components of the TOSA 800. The MLM
source 801 can have an emission spectrum having characteristics as
illustrated by FIG. 4A. The MLM source 801 can be implemented by a
Fabry-Perot laser. In one embodiment, light emitted from the MLM
source 801 is precision coupled to a lens or a collimating device
806.
[0082] A temperature sensor 802 is placed at the vicinity and in
thermal communication with the MLM source 801 to monitor the
operating temperature of the MLM source 801. The MLM source 801 can
be mounted on a carrier plate 804. The carrier plate 804 and the
temperature sensor 802 are mounted on the H/C 805. The H/C 805 can
be controlled by an external signal 815. The H/C 805 can also
control the temperature to a designated set-point in response to
the temperature sensing signal 812 that is output by the
temperature sensor 802 built-in the same unitary device. The H/C
805 can also control the temperature to a designated set-point in
response to an external signal. The H/C 805 can be in the form an
extended stage so it can support and be in thermal contact with
multiple components. The carrier plate 804 and the temperature
sensor 802 are in good thermal contact with the H/C 805.
[0083] In the optical communication system 200, the external signal
815 can be transmitted by the counterpart transmitter at the
opposite end of the communication network. For example, the
transmitters 208-1 and 228-1 can be implemented as the
temperature-controlled TOSA 800. The temperature controller in the
transmitter 228-1 can be controlled by a temperature control signal
sent by the transmitter 208-1, and vice versa. The temperature
control signal sent from the transmitter 208-1 to the transmitter
228-1 can be dependent on the power of the upstream optical signal
sent from the transmitter 228-1 and received by the receiver 210-1.
The interactive temperature controls between the transmitters 208-1
and 228-1 allow the temperatures of the transmitters 208-1 and
228-1 to be tuned such that the emission spectra of the
transmitters 208-1 and 228-1 can be set to substantially the same
wavelength channel.
[0084] In one embodiment, the MLM source 801 is a Fabry-Perot
semiconductor laser 820, as shown in FIG. 8C. The Fabry-Perot
semiconductor laser 820 includes a cavity 830. The cavity 830
includes a front facet 836 and a back facet 837. Each end facet of
the cavity 830 is appropriately coated to reflect the laser light
back and forth in the cavity 830. As the light reflects between the
both ends of the cavity the allowable modes of the MLM source must
satisfy the wavelength condition:
.lamda..sub.m=2.times.L.times.n/m. where n is the refractive index
in the cavity, m is an integer, L is the length of the laser cavity
and .lamda..sub.m is the peak wavelength. Semiconductor materials
usually can emit lights at a wide spectral range. The laser cavity
830 is usually much longer than the wavelengths emitted by the
semiconductor materials. Thus, the emissions can include a
plurality of modes each characterized by a different central
wavelength as shown in FIG. 4A. Each individual spectral peak is
referred as one longitudinal mode and a Fabry-Perot laser exemplify
a typical multi-longitudinal mode (MLM) source.
[0085] In the optical communication system 200, it is sometimes
desirable to broaden the FWHM of spectral envelop 405 of the MLM
sources and increase the temperature coefficient of the central
wavelength. These can be accomplished by controlling the properties
of the semiconductor materials selected in the Fabry-Perot
laser.
[0086] The optical emissions exiting the back facet of the MLM
source 801 can be received and monitored by a photo-diode monitor
803. The photo current of the photo-diode monitor 803 can be used
to produce a signal 813 that is indicative of the optical power of
the MLM source 801.
[0087] Bidirectional OSA is a commonly used component in optical
transceivers. A conventional bidirectional OSA package 900 is shown
in FIG. 9A. The key components in the bidirectional OSA 900 include
a WDM filter 909, a TO-CAN TOSA 907, and a TO-CAN ROSA 905. A
housing block 903 holds all the components together. The output
light from the TO-CAN TOSA 907 directly passes the WDM filter 909,
and is coupled into an external optical fiber at a fiber port 901.
The fiber port 901 may also include a sleeve, a ferrule, a lens,
and a stress relief boot. Usually, the input light signal from the
same fiber port has a different wavelength compare to the output
light from TO-CAN TOSA 907. The input light is reflected by the
filter 909, and is then received by TO-CAN ROSA 905.
[0088] In comparison, FIG. 9B shows the structure of a tunable MLM
bidirectional OSA 920 in accordance with the present specification.
Similar to the bidirectional OSA package 900, the tunable MLM
bidirectional OSA 920 includes a WDM filter 929, a TO-CAN ROSA 925,
a housing block 923, and a fiber port 921. The spectral
distributions of the MLM sources (e.g. 208-1 and 228-1) in the
above described optical communication systems 200 can be tuned
using a temperature controller, as shown in FIGS. 3A, 3B and 8A,
8B. The tunable MLM bidirectional OSA 920 includes a tunable MLM
TOSA 927. A temperature controller can be integrated in the tunable
MLM TOSA 927. The input and output signals of the tunable MLM TOSA
927 should include but not limited to high-speed differential
signals, a temperature sensing signal 812, and a temperature
control signal 815 as outlined above in relation to FIG. 8. ROSA
925 includes normal differential data signals and also an
additional output leads 935 that can provide power level indicator
based on the current of the photodiode in the ROSA 925 (also as
described above in FIG. 7A).
[0089] FIG. 10 is a block diagram of an optical transceiver module
1000 in accordance with the present specification. The transceiver
module 1000 includes a temperature controller 1004, a TOSA 1006, a
light source driver 1008, a ROSA 1003, and a micro-controller unit
(MCU) 1010 having an inter-integrated circuit (12C) interface. The
TOSA 1006 can be a tunable MLM source that is compatible with a
high speed WDM-PON system such as the optical communication system
200.
[0090] The optical transceiver module 1000 has more functions that
are crucial to the implementation of the optical communication
systems 200. The TOSA 1006 can include a transmitter 1006a, and a
heating and cooling device 1006b. The transmitter 1006a can be an
MLM source, such as a Fabry-Perot laser, having an emission
spectrum with characteristics as illustrated by FIGS. 4A-4B. The
light source driver 1008 can be for example a laser driver for the
Fabry-Perot laser. High speed differential signals for light source
driver 1008 are connected with "TD+" and "TD-". The light source
driver can be disabled through the control signal "Dis".
[0091] The operating temperature of the transmitter 1006a can be
monitored by a temperature sensor 1006c. The output signal from the
temperature sensor 1006c can be used by the MCU 1010 to control the
H/C device 1006b that controls the temperature of the transmitter
1006a. The central wavelength of the transmitter 1006a is dependent
on temperature. The transmitter 1006a is thus a low cost light
source whose emission spectrum can be externally tuned by control
its operation temperature. The TOSA 1006 can further includes a
photo detector 1006d that can detect the back-facet emission
intensity of the MLM source and output a MLM power signal to be
received by either the MCU 1010 or the light source driver 1008.
The MLM power signal can be used as a feedback signal to control
the light source driver 1008 to ensure a stable MLM emission in the
specified intensity.
[0092] The ROSA 1003 includes a photo detector (PD) 1003a and a
transimpedance amplifier (TIA) 1003b. In one of the embodiments of
current specification, ROSA 1003 can provide a photocurrent monitor
signal that is approximately proportional to the power of the input
optical signal. The high speed differential output signals from TIA
1003b are in connection with the input ports of a post amplifier
1007. The post amplifier 1007 then provides standard outputs
including the high speed differential received signals (RD+ and
RD-) and signal detect (SD) indicator. The post amplifier 1007 can
also provide a RSSI signal. The power monitor signal that indicates
the power of the input optical signal can be implemented as the
photo-current monitor signal from the TIA 1003b or the RSSI signal
from the post amplifier 1007 can be received and digitized by the
MCU 1010 through an A-D converter. The host (i.e. OLT 102) can
receive the optical power signal through the I2C interface or in
optical signals from a remote optical transceiver module (e.g. at
an ONU 104) and utilize it as a feedback signal to control the
temperature of the transmitter at the remote optical transceiver
module (e.g. ONU).
[0093] The present specification is described above with reference
to exemplary embodiments. It will be apparent to those skilled in
the art that various modifications may be made and other
embodiments can be used without departing from the broader scope of
the present specification. Therefore, these and other variations
upon the exemplary embodiments are intended to be covered by the
present specification.
* * * * *